The photovoltaic effect used in solar cells allows direct conversion of light energy from the sun's rays into electricity by way of the generation and transport inside a semiconductor material of positive and negative electrical charges. The action of light impinging on the semiconductor material creates positive and negative charges unbound, or weakly bound, to each other, that are capable of diffusing to or being otherwise captured by different electrodes in contact with the semiconductor.
Electrodes are placed on both sides of the semiconductor material to collect the electrical charge. Light must enter the solar cell through at least one of the electrodes, generally referred to as the “front” electrode. Thus, the front electrode must be transparent to light as well as be electrically conductive.
Transparent front electrodes typically consist of a silver wire grid pattern that is applied to the surface of the semiconductor material by screen printing or other form of contact printing. Alternatively, they may consistent of a more uniform/contiguous film of transparent conductive material, such as a film of indium tin oxide (“ITO”).
ITO films suffer from a number of disadvantages including inferior transparency, particularly in the infrared and ultraviolet regions of the spectrum, and marginal conductivity. Both of these disadvantages result in lower efficiency of the solar cell. ITO is also expensive and concerns have been raised about dwindling global supplies of indium. ITO is also brittle and does not lend itself to roll-to-roll processing or use in flexible solar cells.
Silver wire grids also have significant drawbacks, especially in the fabrication of solar cells with silicon wafers. The application of the grid pattern to the silicon wafer by contact printing techniques can result in significant wafer breakage. Sensitivity to breakage requires manufacturers to use thicker silicon substrates than might otherwise be preferred, and the thickness of silicon substrates is a dominant factor in overall cell cost. Further, conventional screen printed Ag electrodes tend to have poor geometries, including poor aspect ratios for front electrode purposes, meaning they are relatively wide (casting a large shadow) and relatively short (meaning offering less overall electrical conductance than would be preferred). Further, they cannot be printed in close proximity to each other owing to resolution limits.
Thus a need exists for an improved transparent conductive front electrode for photovoltaic cells that eliminates the disadvantages of the transparent conductive front electrodes currently used.